1. Field of the Invention
The present invention relates to a surface acoustic wave device such as a surface acoustic wave convolver, and a signal receiver and a communication system utilizing the same.
2. Related Background Art
A surface acoustic wave device, particularly a surface acoustic wave convolver, is increasingly considered important as the key device in the spread spectrum communication, and is recently the target of intense development.
FIG. 1 is a schematic view showing the concept of such conventional surface acoustic wave device. There are provided a piezoelectric substrate 81 composed for example of Y-cut(Z-propagation) lithium niobate; comb-shaped electrodes or interdigital transducers (IDT) 82, 83 formed on the surface of the piezoelectric substrate 81; and an output electrode 84 formed on the piezoelectric substrate 81.
These electrodes are composed of a conductive material such as aluminum, and are usually formed by a photolithographic process.
There are also shown a PN code generator 85, and an input circuit 86.
In the surface acoustic wave device of the above-explained structure, an electrical signal of a carrier angular frequency .omega. supplied to the input IDT 82 induces a surface acoustic wave by the piezoelectric effect of the substrate. Similarly an electrical signal of a carrier angular frequency .omega. supplied to the IDT 83 induces another surface acoustic wave. By causing these two surface acoustic waves to propagate in mutually opposite directions on the piezoelectric substrate, a convolution signal (carrier angular frequency 2.omega.)) of the two input signals can be obtained from the output electrode, by means of the physical non-linear effect of the piezoelectric substrate.
On a coordinate system with the x-axis taken along the wave guide path and the original point taken at the left-hand end thereof, the two surface acoustic waves can be represented as: EQU F(t-x/v)e.sup.j(.omega.t-kx), G(t+x/v)e.sup.j(.omega.t+kx)
and the non-linear interaction of these two waves generates, as the product, a surface acoustic wave independent of the location: EQU F(t-x/v).multidot.G(t+x/v)e.sup.j2.omega.t
on the piezoelectric substrate.
By forming a uniform output electrode, this signal is integrated within the length of the electrode, and can be taken out as a signal represented by: EQU S(t)=Ke.sup.j2.omega.t .intg.F(t-x/v).multidot.G(t+x/v)dx (1)
within the length of interaction under the output electrode. The range of integration can be practically considered as .+-..infin. if the length of interaction is sufficiently larger than the signal length, and, by taking .tau.=t-x/v, the equation (1) can be rewritten as: EQU S(t)=-vKe.sup.j2.omega.t .intg.F(.tau.).multidot.G(2t-.tau.)d.tau.(2)
so that said signal becomes the convolution of the input signals.
The mechanism of such convolution is detailedly explained, for example, by Shibayama; "Application of surface acoustic wave", Television, 30, 457(1976).
As explained in the foregoing, the convolver utilizing the surface acoustic waves obtains, by means of the configuration shown in FIG. 1, the convolution output of a detected signal and a reference signal. The output electrode 84 shown in FIG. 1 also serves as a wave guide path utilizing an effect of electrical shortcircuiting of the surface of the piezoelectric substrate when a thin metal film is evaporated thereon, and an effect of concentration of the surface acoustic wave, as the velocity thereof is reduced by the mass load effect of the evaporated metal.
As the efficiency of convolution is proportional to the square of energy density, the output electrode serving also as the wave guide path is required to be as narrow as possible, within an extent that the convolution output can be taken out, in order to prevent-the higher-order transversal propagation modes and to increase the energy density. On the other hand, a narrower wave guide path increases the variation in velocity dependent on the frequency. In consideration of these factors, the width of the wave guide path in a Z-propagation convolver employing Y-cut lithium niobate is generally selected within a range of 1.5.lambda. to 4.lambda., wherein .lambda. is the wavelength of the Z-propagating surface acoustic waves.
On the other hand, in the input IDT for converting the electrical signal into the surface acoustic wave signal, the proportion of conversion of the electrical energy into the energy of surface acoustic wave is determined by the electromechanical coupling coefficient of the substrate. Consequently the number of fingers of IDT has to be increased if the electromechanical coupling coefficient is small.
However, as an increase in the number of fingers of the IDT reduces the band width, there have been conceived various electrode structures with frequency dispersing property, capable of inducing the surface acoustic wave in a wider frequency range.
As described in "Surface acoustic wave device and its application" edited by Electronic Material Industry Association and published by Nikkan Kogyo Shimbun, 3. Delay line; 3.1 Non-dispersing delay line; 3.1.1 Band spreading, an electrode structure with such frequency dispersing property utilizes an IDT having plural pitches for oscillation in plural frequency bands and provided in a direction perpendicular to the propagating direction of the surface acoustic wave.
FIG. 2 is a schematic view showing the shape of the input IDT of a surface acoustic wave convolver, having a structure provided with plural pitches for inducing excitation in such plural frequency bands, and featured by a fact that such structures are positioned in a direction perpendicular to the propagating direction of the surface acoustic waves. In FIG. 2 there are illustrated wire bonding pads 11, 12 (11', 12'); high frequency oscillating IDT portions 13 (13'); intermediate frequency oscillating IDT portions 14 (14'); and low frequency oscillating IDT portions 15 (15').
There is also known so-called chirping electrode in which the pitch of fingers of the IDT electrodes is not constant. U.S. Pat. No. 4,649,509 (1987) describes a convolver with a dispersive input IDT, utilizing such chirping electrodes in which the IDT is shaped as an iso-phase plane of the surface acoustic wave emerging from the wave guide path. However, there is not given a clear explanation on the frequency dispersing means of such frequency dispersive IDT, except that the spacing of the electrode fingers is not constant.
Such conventional chirping electrodes have been associated, however, with the following drawbacks that:
(1) the loss inside the electrodes is large;
(2) the surface acoustic waves of different frequencies, being excited in the different locations on the electrode in the propagating direction of the surface acoustic waves, require different times for reaching the output integrating electrode, so that the peaks of convolution outputs are mutually displaced between the high and low frequencies; and
(3) with an increase in the number of electrode fingers, a next chip in the pseudo noise signal used in the SS communication is excited before a surface acoustic wave, excited in a portion of the IDT distant from the wave guide path, can pass a portion of the IDT close to the wave guide path, so that the signal is integrated over a range longer than the one-chip length of the pseudo noise signal and is therefore blunted, and these drawbacks result in the loss in the convolution efficiency.
Also the IDT as shown in FIG. 2 has a drawback that it cannot concentrate the surface acoustic wave.